Nitrogen retention mechanisms in tokamaks with beryllium and tungsten plasma-facing surfaces

Abstract

Global gas balance experiments at ASDEX Upgrade (AUG) and JET have shown that a considerable fraction of nitrogen injected for radiative cooling is not recovered as N 2 upon regeneration of the liquid helium cryo pump. The most probable loss channels are ion implantation into plasma-facing materials, co-deposition and ammonia formation. These three mechanisms are investigated in laboratory and tokamak experiments and by numerical simulations. Laboratory experiments have shown that implantation of nitrogen ions into beryllium and tungsten leads to the formation of surface nitrides, which may decompose under thermal loads. On beryllium the presence of nitrogen has been seen to reduce the sputtering yield. On tungsten surfaces it has been observed that the presence of nitrogen can increase hydrogen retention. The global nitrogen retention in AUG by implantation into the tungsten surfaces saturates. At JET the steady state nitrogen retention is increased by co-deposition with beryllium. The tokamak experiments are interpreted in detail by simulations of the global migration with WallDYN. Mass spectrometry of the exhaust gas of AUG and JET has revealed the conversion of nitrogen to ammonia at percent-levels. Conclusions are drawn on the potential implications of nitrogen seeding on the operation of a reactor in a deuterium-tritium mix.

Evolution of nitrogen concentration and ammonia production in N2-seeded H-mode discharges at ASDEX Upgrade

“…As has been previously reported [4,7,10,11], the mass spectra of ammonia are expected to overlap with those of methane and water, which are also present in the neutral phase during the discharge phases, with varying ratios of hydrogen and deuterium atoms. In order to distinguish between the contributions of the three impurities to the mass spectrometer recordings, a statistical model, described in detail in [7,11] has been fit to the data.…”

“…In all three locations, for the non-seeded discharges, the fit reported non-zero pressures of ammonia. As no ammonia formation was expected in a nonseeded discharge [4,7,13,14], the reported ammonia pressures from the non-seeded discharge can serve as an estimate of the reliability of the detection.…”

“…It is retained in the plasma-facing surfaces by ion implantation and formation of W x N and Be x N [4] which results in a noticeable build-up and legacy of the nitrogen content in the plasma [5,6]. Most importantly, nitrogen seeding has been shown to lead to ammonia production [4,[6][7][8], which could be a potential issue for ITER. Nitrogen seeding in the active phase of ITER operations would lead to formation of partially tritiated ammonia.…”

“…Residual gas analysis of discharges with nitrogen seeding at JET shows that formation of ammonia increases with the amount of seeded nitrogen, however large scatter indicates that other parameters could also influence the ammonia formation [6]. In first dedicated ammonia formation experiments at JET [4], it was shown that in a series of discharges with the same or very similar rates of nitrogen seeding, ammonia formation exhibits a significant evolution, increasing continuously both in the timeframe of a single discharge as well as from one discharge to another. In both devices, a significant legacy effect of nitrogen was observed, i.e.…”

Evaluation of the plasma hydrogen isotope content by residual gas analysis at JET and AUG

“…Due to the need to ensure tritium self-sufficiency, the vessel in the future fusion devices will be surrounded by the breeding blanket, which will severely limit the access to the plasma. Residual gas analysis (RGA) has already been shown to provide information about certain plasma-wall interaction processes in fusion devices (5,6). As it does not require direct access to the plasma, nor immediate vicinity to the plasma containing vessel, it is a suitable candidate for use in future, diagnostic-limited fusion devices.…”